.myflex {
  /* 将元素高宽设置成浏览器可视区域高宽 */
  width: 100vw;
  height: 100vh;

  display: flex;
  justify-content: center;
  align-items: center;
}

.myflex > div {
  border: 1px solid #ff0000;
  margin-top: -10rem;
}
